An alternative succinate (2-oxoglutarate) transport system in Rhizobium tropici is induced in nodules of Phaseolus vulgaris.

The rhizobial DctA permease is essential for the development of effective nitrogen-fixing bacteroids, which was correlated with its requirement for growth on C(4)-dicarboxylates. A previously described dctA mutant of Rhizobium tropici CIAT899, strain GA1 (dctA), however, was unexpectedly still able to grow on succinate as a sole carbon source but less efficiently than CIAT899. Like other rhizobial dctA mutants, GA1 was unable to grow on fumarate or malate as a carbon source and induced the formation of ineffective nodules. We report an alternative succinate uptake system identified by Tn5 mutagenesis of strain GA1 that was required for the remaining ability to transport and utilize succinate. The alternative uptake system required a three-gene cluster that is highly characteristic of a dctABD locus. The predicted permease-encoding gene had high sequence similarity with open reading frames encoding putative 2-oxoglutarate permeases (KgtP) of Ralstonia solanacearum and Agrobacterium tumefaciens. This analysis was in agreement with the requirement for this gene for optimal growth on and induction by 2-oxoglutarate. The permease-encoding gene of the alternative system was also designated kgtP in R. tropici. The dctBD-like genes in this cluster were found to be required for kgtP expression and were designated kgtSR. Analysis of a kgtP::lacZ transcriptional fusion indicated that a kgtSR-dependent promoter of kgtP was specifically induced by 2-oxoglutarate. The expression of kgtPp was found in bacteroids of nodules formed with either CIAT899 or GA1 on roots of Phaseolus vulgaris. Results suggested that 2-oxoglutarate might be transported or conceivably exported in nodules induced by R. tropici on roots of P. vulgaris.

Laccase activity in melanin-producing strains of Sinorhizobium meliloti.

Laccase-like activity was detected in melanin-producing strains of Sinorhizobium meliloti mainly in cells at the stationary growth phase when copper was added to the medium. The laccase showed both syringaldazine and ABTS (2,2'-azino-bis-ethylbenzthiazoline-6-sulfonic acid) oxidase activities and was activated by the addition of 1.7 mM sodium dodecyl sulfate. Activity was totally inhibited by the addition of 1.0 mM EDTA, suggesting that the enzyme is a metal-dependent one. The enzyme was found to be cytosolic having an optimum pH of 5.0, an estimated molecular mass of 95 kDa and a K(m) of 4 microM for syringaldazine. Both laccase and tyrosinase activities were detected in melanin-producing S. meliloti strains. Plant growth-promoting (PGP) effect in rice by a laccase-producing S. meliloti strain when co-inoculated with Azospirillum brasilense Cd was observed. PGP effect by co-inoculation significantly increased plant yield compared to A. brasilense by itself. To the best of our knowledge this is the first report on laccase production in rhizobia and cooperation between Azospirillum and Sinorhizobium in rice.